The immune system attempts to fight a transplanted organ in the same way it fights an infection or a cancer. Without immunosuppressive medication to inhibit the immune system's action, a transplanted organ is quickly rejected and stops functioning. Organ transplant recipients can experience some organ rejection even when they are taking immunosuppressive drugs. Rejection occurs most frequently in the first few weeks after transplantation, but rejection episodes can also happen months or even years after transplantation. Combinations of up to three or four medications are commonly used to give maximum protection against rejection while minimizing side effects. Current standard drugs used to treat the rejection of transplanted organs interfere with discrete intracellular pathways in the activation of T-type or B-type white blood cells. Examples of such drugs are cyclosporin, daclizumab, basiliximab, everolimus, or FK506, which interfere with cytokine release or signaling; azathiopirene or leflunomide, which inhibit nucleotide synthesis; or 15-deoxyspergualin, an inhibitor of leukocyte differentiation. The beneficial effects of these therapies relate to their broad immunosuppressive effects; however, the generalized immunosuppression which these drugs produce also diminishes the immune system's defense against infection and malignancies. Furthermore, standard immunosuppressive drugs are often used at high dosages and can themselves cause or accelerate organ damage in either the transplanted organ itself, or in other target organs of the transplant recipient.